Chemistry Reference
In-Depth Information
The Color of Fired Pottery
Primary clay, for example kaolin, is colorless, and when such clay is heated
to a high temperature it produces white ceramic materials. Most pottery,
however, is colored: its color is due to the fact that most of it was, and still
is, made not from primary but from secondary clay. Secondary clay contains
minerals other than clay, and colored metal ions in them endow the pottery
with their color. Iron ions (in iron oxides), for example, tend to make pottery
yellow, brown, or red, and manganese ions (in
pyrolusite
, a mineral com-
posed of manganese oxide) make it either dark or black.
The atmosphere in the kiln, as well as the temperature at which pottery
is fired, also affects its final color. A draft in the kiln ensures an abundant
supply of oxygen and facilitates the oxidation of the metals. When pottery
containing iron, for example, is fired under oxidizing conditions, the iron is
gradually oxidized and
baked pottery
(fired below 800°C) acquires a
buff color; but if it the pottery is
burned
(fired above 850°C), more oxidized
iron ions generally cause it to be yellow or red. If the kiln's draft is shut off,
however, the atmosphere in the kiln becomes filled with reducing gases
(such as hydrocarbons, released by wood in the early stages of the
combustion, and carbon monoxide, formed when organic matter is heated
in an atmosphere lacking in oxygen). These gases prevent the oxidation of
the organic matter additives in the clay paste, which is therefore converted
into carbon particles. As a consequence, and depending on the amount of
organic matter in the clay paste, pottery fired in a reducing atmosphere is
either gray (when there is little organic matter) or totally blackened by the
deposition of
soot
(unburned carbon) particles within the fabric of the fired
clay. Often, black pottery is made by coating the walls of dry clay objects,
before firing, with a layer oil or bitumen. The oil or bitumen initially per-
meates into the walls of the objects; when these are then fired in a kiln with
a reducing atmosphere, the oil or bitumen is charred, forming extremely
small particles of
soot
that impart to the surface of fired objects a gray or
black finish.
Glazing
The surface of fired pottery is generally porous and readily absorbs mois-
ture. To conceal outside imperfections, decorate the surface, and make the
pottery more or less impervious to water and other liquids, many ceramic
objects are
glazed
, that is, coated with a thin layer of
glaze
(Tite 2004). The
nature of glaze and the glazing process are discussed in Chapter 3.
